So Pharaoh’s heart was hardened, and he would not let the Israelites go, just as the LORD had said through Moses.
The threatened plague of hail is here summoned by the powerful hand and rod of Moses (Exo 9:22, Exo 9:23), and it obeys the summons, or rather the divine command; for fire and hail fulfil God's word, Psa 148:8. And here we are told, I. What desolations it made upon the earth.
Commenting on Exodus 9:22-35
PLAGUE OF HAIL. (Exo. 9:18-35) I will cause it to rain a very grievous hail, &c.--The seventh plague which Pharaoh's hardened heart provoked was that of hail, a phenomenon which must have produced the greatest astonishment and consternation in Egypt as rain and hailstones, accompanied by thunder and lightning, were very rare occurrences.
Commenting on Exodus 9:18-35
And the heart of Pharaoh was hardened - In consequence of his sinning yet more, and hardening his own heart against both the judgments and mercies of God, we need not be surprised that, after God had given him the means of softening and repentance, and he had in every instance resisted and abused them, he should at last have been left to the hardness...